Chemistry Reference
In-Depth Information
positive at the shortest displayed O
O distance. In brief, the complex variation
occurring to shape, size, and Laplacian distribution of the H atom along the reaction
coordinate R
O
O
is nicely summarized by and translated into the change of a single-
valued function of R
O
O
.
When the SF tool was applied to a series of prototypical HB complexes, most of
the qualitative conclusions from the model study on water dimer were retained, but
important new and subtle facets also emerged [
9
]. The investigated complexes are
shown in Fig.
4c
and classified according to Gilli and Gilli [
48
] as charge-assisted
hydrogen bonds, [
OH
2
)
þ
; -CAHB,
2
: the open form of the
formic acid-formate anion complex], resonance-assisted hydrogen bonds [RAHB,
malonaldeyde, in its C
s
equilibrium form,
3
, and in its C
2v
transition state,
4
, for the
H-atom transfer between the two oxygen atoms], polarization-assisted hydrogen
bond [PAHB;
5
: cyclic homodromic
9
water trimer], and isolated hydrogen bond
[IHB;
6
: water dimer at equilibrium geometry]. The SF percentage contributions
listed in Table
6
and displayed in Fig.
4c
neatly confirm S%(H) as a clear indicator
of the H-bond nature. Its value shows even more pronounced variations with change
in the donor to acceptor atom distance than found for corresponding distance
changes along the approach of two water molecules. Enhanced changes with
increasing or decreasing R
O
O
are similarly found for the percentage sources
from other atoms or group of atoms. For instance, S%(H) and S%(H
þ
CAHB,
1
:(H
2
O
H
A) are already
as large as about 32% and 82% for systems
1
,
2
at an R
O
O
distance of about 2.4
˚
,
while they amount to about only 3% and 49% in the water dimer at the much shorter
2.25
˚
þ
R
O
O
distance. The percentage source from the two H-bonded atoms,
S%(H
þ
A), clearly denotes a marked covalent character for this bond in the
CAHB systems, while it is not so for the neutral water dimer system when com-
pressed at similar or even shorter donor to acceptor atom separations. At variance
with the compressed water dimer,
CAHB complexes exhibit equal or almost
equal O-H and H
O distances, and it is this symmetric location of the H involved
in the HB that leads to the enhanced percentage source contribution from H and to
the pronounced covalent character of their HBs.
Analysis of the SF contributions for the RAHB in malonaldeyde, 3, revealed
quite an interesting fact [
9
]. The sum of percentage SF contributions from the triad
of atoms directly involved in the RAHB did not follow the trend one would have
expected from the HBs in the other members of the series, but rather showed a
value, S% (H
70.8, which is comparable to that found in the cyclic water
trimer,
5
, or in the water dimer at R
O
O
2.750
˚
, despite R
O
O
being in 3 shorter by
0.250
˚
. A similar S%(H
þ
D
þ
A)
¼
A) contribution decrease, with respect to the
expected trend, was also observed for the RAHB in the malonaldeyde TS,
4
. Its S
%(H
þ
D
þ
A) value was found to be comparable and even slightly smaller than that
for CAHBs
1
and
2
, despite their larger O
þ
D
þ
O separations.
9
A cyclic (H
2
O)
n
water polymer is called sequential or homodromic if each O atom is acting
contemporarily as an H-donor and as an H-acceptor.
